The invention relates to a system for quantitative determination of the areal distribution of a quantity to be measured, including a planar sensor film, which is applied to a possibly curved measuring surface, such as the surface of a body organ, a region of the skin or a cell culture, and whose diffusion properties regarding a parameter to be determined are known, said sensor film containing a luminescent indicator responding to the parameter to be determined with a change of at least one optical property, and comprising an excitation and detection unit including a means for supplying excitation radiation of at least one wavelength and an imaging detection means, preferably a CCD camera, and an evaluation unit processing the image information obtained from the excitation and detection unit.
In medical diagnosis and in the monitoring and control of medical treatment it may be most desirable to determine the areal distribution of the concentration of a measurement variable on the surface of a body organ, such as the skin, or the distribution of the flow rate of a given substance through a boundary surface, such as a surface of a body organ, a region of the skin, or a cell culture in a laboratory set-up.
In this context a schematical configuration has become known from EP 0 516 610 B1, by means of which readings for the material flux, such as oxygen flux, through an interface, such as a skin surface, may be taken. The sensing layer of the measuring device, which is assigned to the boundary face or measuring surface, will exert a known or predefined, finite resistance to the material flux to be determined, at least one optical indicator being provided in the sensing layer for determining the material concentration on one side of the sensing layer. The measured or known concentration value on the other side of the sensing layer, or the difference relative to the first concentration value, is used to determine the material flux through the boundary face. In a variant of the invention the surface of the sensing layer is scanned by a plurality of detectors or by means of an imaging system (CCD).
In U.S. Pat. No. 5,593,899 an apparatus and method for measuring tissue oxygenation are disclosed, using the oxygen dependent quenching of a fluorescent indicator. To measure oxygen supply, a luminescent layer, which is part of a skin cream, is applied to a particular surface region of the skin, and is covered with an oxygen-impermeable film. The optical device enclosed in a plastic housing is provided with a plastic or glass cover which is directly placed over the O2-impermeable film. Further included are an interference filter and a photodiode. Via optical fiberguides the luminescent layer is exposed to excitation radiation from a modulated radiation source. The plastic housing further includes heating elements, which are connected to a thermoregulation circuit and heat up the measuring region to a temperature of 39xc2x0 to 42xc2x0 C. The arrangement described in this document is suitable only for integrated measurements over the entire measuring region covered by the optical device. It does not yield accurate information on boundaries between regions with satisfactory oxygen supply and those with oxygen deficiency.
Theoretical considerations on the local distribution of oxygen flux or subcutaneous oxygen concentration and a proposal concerning an imaging technique are disclosed in xe2x80x9cFluorescence Lifetime Imaging of the Skin PO2: Instrumentation and Resultsxe2x80x9d in Advances in Experimental Medicine and Biology, Vol. 428, 605-611 (1997), Plenum Press, N.Y. In this article a sensor membrane measuring transcutaneous oxygen concentration is described, which consists of an optical isolating layer next to the skin, a sensing layer containing a luminescent indicator with O2-sensitive decay time, and an oxygen-impermeable supporting foil. Another membrane measuring oxygen flux, which is also described in this paper, differs from the former by a diffusion barrier with known oxygen permeability, which is used instead of the O2-impermeable layer. For the measuring process the sensor membrane is applied to the measuring surface, for example, a region of skin. A modulation technique is employed for measuring, where the LEDs emitting excitation radiation in the direction of the sensor membrane are driven by a square-wave generator. The phase-shifted light radiation emitted by the sensor membrane is detected by a CCD camera with modulated amplification and transmitted pixel by pixel to a computing unit for image-processing. Images of the oxygen distribution measured in a polymer layer as compared to the inhomogeneous oxygen distribution over a region of the skin are part of the documentation.
Other imaging methods using phase fluorimetry are disclosed in U.S. Pat. No. 5,485,530.
Finally, the paper xe2x80x9cA modular luminescence lifetime imaging system for mapping oxygen distribution in biological samplesxe2x80x9d published in Sensors and Actuators B 51 (1998) 163-170, is concerned with an imaging method measuring two-dimensional oxygen distributions with planar optodes. The experimental set-up includes a test chamber with several perfusion channels, through which water with different oxygen concentrations is guided. In the direction of the optical system the channels are closed with a planar optode pressed against the test chamber. For excitation of the planar optode a fiber-optical ring light source is employed, radiation detection is effected by means of a CCD camera.
It is an object of this invention to further develop the above system so as to obtain reproducible measurement results of the areal or local distribution of the concentration of a measurement variable over a possibly curved surface, or the areal distribution of the material flux through a possibly curved surface, and to display the obtained results with the use of an imaging method.
According to the invention this object is achieved by providing the apparatus with an applicator tube to be placed over the measuring surface, whose rim facing the measuring surface is in elastic contact with the sensor film and is impervious to external radiation interfering with the detection means.
Following is a more detailed discussion of the present invention which is exemplified by measurements of the areal distribution of the transcutaneous oxygen concentration and the areal distribution of the oxygen flux through the skin, this does not imply that the invention is restricted to this specific purpose, the advantages obtained by this procedure will be fully applicable in the case of other medical or biological quantities and parameters. With a suitably chosen luminescent indicator it will be possible, for example, to measure the subcutaneous CO2 concentration or distribution of the CO2 flux through the skin. The system of the invention could also be employed for determining the areal distribution of the concentration or flux of ions, if the sensor film which is applied to the measuring surface, includes a luminescent indicator responding to the ion in question.
Use of the applicator tube to be placed on the measuring surface will result in an unambiguously defined optical configuration yielding reproducible measurement results. With its elastic rim that is in elastic contact with the sensor film, and its constant distance to the measuring surface given by the length of the tube, the applicator tube serves to protect the sensitive detection unit from straylight or undesirable background radiation. This is also true for uneven surfaces onto which the elastic rim fits closely.
The system is particularly suitable for use with curved measuring surfaces, for example, for measuring subcutaneous oxygen concentrations in a patient""s limbs, where the detection means preferably includes a unit for determining the local fluorescence decay time or a derived quantity. To obtain an imaging technique that is independent of inhomogeneities of the distribution of the luminescent indicator in the sensor film, and of inhomogeneous optical conditions, the invention preferably relies on decay time measurement. A special CCD camera is operated in such a way that the information in each pixel can be correlated with the decay time prevailing at the measuring site corresponding to the pixel. The special advantage of measuring a parameter that is exclusively defined by the decay time of the indicator, is that the measurement will become independent of the local illuminance at the sensor film. As a consequence, it will be possible to illuminate even curved surfaces by means of a simple configuration of the excitation light source, without having to provide means for homogeneous illumination, as long as sufficient luminescence radiation is generated for decay time measurement in each measuring point. Since decay time measurement, as opposed to intensity measurement, is not subject to any dependence on the square of the distance of the measured object, and since different propagating times of the light due to different distances of the measured object are negligible from the point of measuring accuracy at the modulation frequencies used in this context, decay time measurement is most suitable for measurements involving the arms or legs, or other curved areas of a patient""s skin.
It is provided in a preferred variant of the invention that the applicator tube be provided at its sealable end facing away from the measuring surface with a coupling means for attaching the excitation and detection unit.
According to the invention the applicator tube may be covered at the end facing away from the measuring surface by a cover plate that is transparent to the excitation and measurement radiation, i.e., a glass plate preferably, or by a removeable cap. This is particularly useful if the sensor film or the measuring region must be thermoregulated prior to measuring until a constant temperature is reached, which process may take several minutes.
In a first variant of the invention the applicator tube may be have inlet and outlet openings through which a heat transfer medium, preferably hot air, is introduced and carried off for thermoregulation of the sensor film. By heating the skin area to temperatures  greater than 33xc2x0 C., and preferably 37xc2x0 C., the activity of the skin is increased. This will reduce the waiting time required until the oxygen concentration at the interface between skin and sensor film has reached its equilibrium with the O2 concentration in subcutaneous layers of tissue. It is only after this delay that the sensor will measure the oxygen concentration corresponding to the subcutaneous oxygen partial pressure. Introducing thermoregulation will reduce the preparation time for the actual measurement, and thus will relieve both patient and clinical staff. Due to the use of the applicator tube described by the invention the patient is not connected to the excitation and detection unit during the adjustment or waiting period, and is thus capable of moving more or less freely. The excitation and detection unit of the system will have to be attached to the coupling means of the applicator tube only just before measurement.
According to the invention the applicator tube could also be provided with a radiation- or heating-element for thermoregulation of the sensor film, such as an infrared light source or an electric heater coil.
As regards the applicator tube the planar sensor film is not subject to any restrictions, and could be provided as a sensor paste applied to the measuring surface, or a sensing foil, or a spray layer.
One and the same type of system as described by the invention may be employed for measuring both concentration or partial pressure and flux, the only distinction being the use of different sensor films. Concentration or partial pressure are measured by means of a sensor film whose side facing away from the measuring surface exhibits a barrier layer that is impermeable to the parameter to be determined. Flux measurements, on the other hand, employ a sensor film which may include additional supporting and filling layers consisting of materials which exhibit a known, finite resistance to the passage of the parameter to be determined.
In a preferred variant of the invention the proposal is put forward that a flexible layer be provided between the sensor film and the measuring surface to bridge the gap between sensor film and measuring surface, i.e., preferably an adhesive layer which is permeable to the parameter to be determined. This will permit the efficient reduction of transverse diffusions of the measured parameter as compared to the rapid diffusion in an air gap, and an improvement of the areal resolution of the measurements. A further advantage is obtained by providing that the flexible layer between sensor film and measuring surface be impervious to radiation interfering with the detection unit (such as the intrinsic fluorescence of the skin).
To permit proper alignment of the excitation and detection unit, the latter is designed such that it can be attached to the coupling means of the applicator tube so as to be rotatable about its optical axis.
If the apparatus is to be fastened to a patient""s arms or legs, the applicator tube can be fixed at the measurement site by means of a cuff.
For measurement of the oxygen concentration or oxygen flux a luminescent indicator should be immobilized in the sensor film, or the sensor film should be coated with a luminescent indicator whose local luminescence quenching is a unique function of the local oxygen concentration.